Heat resistant reinforced composites of copolyimides

ABSTRACT

Copolyimides are prepared from pyromellitic dianhydride (PMDA) or mixtures of pyromellitic dianhydride with 3,3&#39;&#39;,4,4&#39;&#39;benzophenone tetracarboxylic acid dianhydride (BTDA), and mixtures of 4,4&#39;&#39;-methylenebis (phenyl isocyanate) (MDI) and toluene diisocyanate (TDI) (2,4-, or 2,6-isomer or mixtures thereof). The mixture of anhydrides is used in a molar percent ratio of PMDA to BTDA of 100 to 75/25 respectively. The mixture of isocyanates is used in a molar percent ratio from about 10/90 to 35/65 of MDI to TDI respectively. The copolyimides of the invention are soluble in their organic reaction solvents but retain their high thermal stability. The solubility allows for the facile preparation of films, laminates, coatings, fibers, and the like. The improvement in heat stability over previously known polyimides is achieved without any loss in solubility properties. The copolyimides can be used in the preparation of high temperature resistant polymer articles and in the various applications for which polyimides are known to be especially adapted.

United States Patent 1 Farrissey, Jr. et a1.

[ 1 Mar. 11, 1975 1 1 HEAT RESISTANT RElNFORCED COMPOSlTlES 0FCOPOLYTMIDES I76] lnventors: William .1. Farrissey, .lr., Northford;Philip S. Andrews, New Haven, both of Conn.

] Filed: Nov. 5, 1973 {21] Appl. No.: 412,976

Related 1.1.5. Application Data [62] Division ofSer. No. 310,398, Nov.29, 1972, Pat. No.

[52] 11.5. C1 260/37 N, 161/197, 161/227, 260/30.2, 260/306 R, 260/308DS, 260/32.4, 260/326 N, 260/65, 260/77.5 R,

260/78 TF [51] Int. Cl. C08g 51/04, C08g 51/16 158] Field 01 Search260/37 N, 65, 77.5 R; 161/197, 227

[56] References Cited UNITED STATES PATENTS 3.300.420 1/1967 Frey260/2.5 3,347,808 10/1967 Levine et a1. 260/29.1

3,422,061 1/1969 Gall 260/47 3.489.696 1/1970 260/2.5

3.546.175 12/1970 Angelo 260/65 3,666,709 5/1972 Suzuki et a1. 260/3343,708,458 l/1973 Albcrino et a1 260/65 3,787,367 1/1974 Farrissey, Jr.et a1. 260/65 Primary Examiner-Lester L. Lee Attorney, Agent, orFirm.1ames S. Rose [57] ABSTRACT Copolyimides are prepared frompyromellitic dianhydride (PMDA) or mixtures of pyromellitic dianhydridewith 3,3,4,4'-benzophenone tetracarboxylic acid dianhydride (BTDA). andmixtures of 4.4- methylenebis (phenyl isocyanate) (MD!) and toluenediisocyanate (TDl) (2,4-, or 2,6-isomer or mixtures thereof). Themixture of anhydrides is used in a molar percent ratio of PMDA to BTDAof 100 to 75/25 respectively. The mixture of isocyanates is used in amolar percent ratio from about 10/90 to 35/65 of MDl to TDIrespectively. The copolyimides of the invention are soluble in theirorganic reaction solvents but retain their high thermal stability. Thesolubility allows for the facile preparation of films, laminates,coatings, fibers, and the like. The improvement in heat stability overpreviously known polyimides is achieved without any loss in solubilityproperties. The copolyimides can be used in the preparation of hightemperature resistant polymer articles and in the various applicationsfor which polyimides are known to be especially adapted.

4 Claims, N0 Drawings HEAT RESISTANT REINFORCED COMPOSITES OFCOPOLYIMIDES CROSS REFERENCE TO RELATED APPLICATION This application isa division of our copending application Ser. No. 310,398, filed Nov. 29,1972, now US. Pat. No. 3,787,367.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to novel polymers and is more particularly concernedwith novel copolyimides which are soluble in their organic reactionsolvents and a process for their preparation.

2. Description of the Prior Art The preparation of insoluble, hightemperature resistant, polyimides from benzophenone-3,3',4,4-tetracarboxylic acid dianhydride, or pyromellitic dianhydride andvarious diisocyanates or the corresponding diamines is well known in theart as in US. Pat. Nos. 3,179,630 and 3,179,631 and 3,562,189 and Srooget al., J. Polymer Science Part A, Vol. 3, pages 1,373 to 1,390, 1965.Soluble copolyimides have been disclosed in a copending application Ser.No. 124,958, filed Mar. 16, 1971 by L. W. Alberino et al. If thepolyimide is prepared via the amide-acid route, namely by reaction ofdianhydride with a diamine it is necessary, in order to make usefularticles such as films, fibers, or shaped articles, to preform thedesired article as the amideacid then convert to the imide by a heat ora chemical treatment. If the polyimide is prepared via the reaction ofdianhydride with diisocyanate, the polyimide usually precipitates fromreaction solution. Generally speaking the final polyimide polymer is noteasily workable in its final form.

The present invention provides a solution to the above problems bydescribing polyimides that are soluble in their reaction solvents andyet retain all their high temperature resistant properties when formedinto finished articles. A further object of the present invention is toprovide soluble polyimides of even greater stability to hightemperatures than previously known soluble polyimides.

SUMMARY OF THE INVENTION This invention comprises novel copolyimidescharacterized by the presence of recurring units of the formula:

wherein the radical R represents in 75-100 percent of said recurringunits and R represents wherein X is a member selected from the groupconsisting of CO, 0, S0 in the remaining 0-25 percent otsnid units; andwherein the radical R represents in 10-35 percent of said recurringunits and in the remaining 65-90 percent of said units R is a memberselected from the group consisting of CH; CH,

and mixtures thereof.

The copolyimides of the present invention are soluble as prepared inorganic solvents, yet retain their excellent high temperature propertieswhen made into films, moldings, coating compositions, laminates andfibers. Due to their solubility the copolyimides of the presentinvention can be easily worked into finished articles.

DETAILED DESCRIPTION OF THE INVENTION The novel copolyimides of theinvention can be prepared by any of the methods known in the art for thepreparation of polyimides from the appropriate anhydride and theappropriate polyisocyanate or corresponding polyamine. lllustratively,the copolyimides of the invention can be prepared by reacting theappropriate mixture of pyromellitic dianhydride (PMDA) andbenzophenone-3,3',4,4'-tetracarboxylic acid dianhydride (BTDA) with theappropriate mixture of di(4- aminophenyl)methane (MDA) and toluenediamine (TDA) (2,4-isomer or 2,6-isomer, or a mixture thereof), or withone of said diamines followed subsequently in the reaction by the otherof said diamines, to obtain the corresponding polyamide acid accordingto the following equation:

cmm-tv-na wherein R and R have the significance above defined.Advantageously, the reactants are brought together in the presence of aninert solvent, i.e., a solvent which does not react with either of thereactants nor interfere in any way with the desired course of thereaction. Advantageously the inert solvent is a dipolar aprotic solvent.Examples of such solvents are dimethylformamide, dimethylacetamide,dimethylsulfoxide, dimethylsulfone, hexamethylphosphoramide, N-methyl-Z-pyrrolidone, tetramethylurea, pyridine, and the like.

Illustrative of the dianhydrides suitable for use in the presentinvention are: pyromellitic dianhydride, 3,3,4,4-benzophenonetetracarboxylic acid dianhydride, bis(3,4-dicarboxyphenyl) etherdianhydride, and bis(3,4-dicarboxyphenyl) sulfone dianhydride.

The reaction of the amines and the anhydride is preferably conductedunder anhydrous conditions and at temperatures below 50C but in somecases temperatures up to 175C may be employed. The desired polyamideacid generally is soluble in the reaction mixture and can be isolatedtherefrom if desired, by conventional methods such as by evaporation ofthe reaction solvent or precipitation by a non-solvent. The amide acidis then converted to the desired polyimide by dehydration. Thedehydration can be accomplished readily by treating the amide acid withan acid anhydride such as acetic anhydride, propionic anhydride, benzoicanhydride and the like, preferably in the presence of a tertiary aminesuch as pyridine, N,N- dimethylaniline and the like. The ring closure isadvantageously conducted at elevated temperatures of 200C or higher.Alternatively, the ring closure of the polyamide acid to the desiredpolyimide can be effected by heat alone.

Illustrative of references which describe detailed conditions forcarrying out the above reactions are U.S. Pat. Nos. 3,179,630 and3,179,631 and Sroog et al., J. Polymer Science Part A, Vol. 3, pages1,373 to 1,390, 1965.

Preferably the copolyimides of the invention are prepared by reaction ofthe appropriate molar proportions of a single dianhydride or mixtures ofdianhydrides with the appropriate molar proportions of 4,4-methylenebis(phenyl isocyanate) (MDI) and toluene diisocyanate (TDI)(2,4-isomer or 2,6-isomer or mixtures thereof) in the presence of adipolar aprotic solvent. Illustrative of the latter solvents aredimethyl sulfoxide, dimethylacetamide, hexamethylphosphoramide,N-methyl-2-pyrrolidone, tetramethylurea, pyridine and the like. Thereaction takes place readily when the reactants are brought together atambient temperatures but elevated temperatures, up to about 160C can beemployed, if desired, in order to increase the rate of reaction. Incarrying out the reaction the dianhydride(s) can be brought togetherwith the mixture of methylenebis(phenyl isocyanate) and toluenediisocyanate so that reaction between the anhydride and each isocyanateoccurs simultaneously with production of an essentially randomcopolyimide.

However, this is not to say some block copolyimide could not be presentbecause of the difference in the reaction rates of the two isocyanates,the methylenebis(phenyl isocyanate) being the more reactive of the two.This constitutes the easiest mode of carrying out the reaction as themethylenebis(phenyl isocyanate) which is normally a solid can bedissolved in the liquid toluene diisocyanate and both addedsimultaneously as a solution. However, if for some reason a blockcopolyimide is desired then the isocyanates can be allowed to reactseparately. The precise composition of the copolyimide, i.e., the ratioof numbers of recurring units corresponding to the imides from toluenediisocyanate and methylenebis(phenyl isocyanate), derived from theappropriate anhydrides is controlled by selecting the appropriate molarproportions of the diisocyanates and dianhydrides used in the abovereaction. Whatever the relative proportions of the isocyanates anddianhydrides which are employed. the total amount of isocyanatesemployed in the reaction is such as to be substantially equimolar withrespect to the dianhydrides.

The copolyimides of the invention can also be prepared by reactingsomething less than the stoichiometric amount of the diisocyanate in afirst stage with the dianhydride, e.g. of the diisocyanate; followed ina second stage by the stoichiometric amount of the diamine correspondingto the diisocyanate necessary to complete the polymerization. Ringclosure of the amicacid linkages is easily accomplished by distilling acombination of the reaction solvent and the formed water from thepolymerization under reduced pressure. The choice of which diamine touse is purely one of convenience and not limited in any way. Thistechnique can sometimes result in a more controlled approach forreaching high molecular weight polymers with less chance for sidereactions. When attempting to polymerize the last remaining anhydridegroups, an excess of diamine in the reaction mixture can be toleratedmuch more readily than an'excess of isocyanate groups which are subjectto trimerizing side reactions.

The amount of dipolar aprotic organic solvent em ployed in the abovedescribed reaction is advantageously' at least sufficient to ensure thatall the reactants are in solution initially. Advantageously, the amountof solvent employed is such that a final polymer concentration of 5% 25%by weight is obtained. The upper limit on the amount of solvent employedis dictated purely by economic considerations. The lower limit on theamount of solvent employed is dictated by the viscosity and resultantproblems in processing a viscous solution. However, the preferred rangeis 10% 15% by weight.

In general, the desired copolyimide of the invention is soluble in thereaction mixture as it is formed in the above described reaction.Copolyimides having such solubility are readily precipitated from thefinal reaction product by addition of a solvent such as acetone,tetrahydrofuran, methylethylketone, chloroform, xylene, benzene, hexane,and the like in which the copolyimide is insoluble. The copolyimides soobtained can be purified, if desired, by washing with appropriatesolvents in which impurities, such as unreacted starting materials, aresoluble.

However, one of the outstanding advantages of the copolyimides of thepresent invention is their solubility in the reaction mixtures as theyare formed. Therefore, if the copolyimide is to be used in a coatingapplication, as a lacquer, or in a laminating process, then there is noneed to isolate the copolyimide from solution. An object of theinvention is to provide coating solutions of the copolyimideshereinbefore described and in the useful concentration rangeshereinbefore set forth.

When isolated, the copolyimides are generally obtained as powders orcoarse solid materials. In order that they can be fabricated into usefulhigh temperature resistant articles, such as those which are commonlyprepared from polyimides, it is necessary to mold the copolyimide. Thisis accomplished generally by converting the copolyimide to a fine powderand subjecting the latter to molding using techniques conventionallyemployed in molding powdered metals such as by sintering or hotpressing; see, for example, Encyclopedia of Chemical Technology, editedby Kirk and Othmer, Interscience Encyclopedia, Inc., Vol. 11, pages54-55, New York, 1953.

It is in the behavior on molding that at least one of the highly usefulproperties of the copolyimides of the invention is manifested. Thus, thecopolyimides of the invention exhibit markedly better flow properties onmolding than do the corresponding polyimides made frommethylenebis(phenyl isocyanate) alone. Further, the higher glasstransition temperatures of the copolyimides of the invention, ascompared with the polyimides derived from methylenebis(phenylisocyanate) alone, means that a corresponding increase in hightemperature stability is achieved. As pointed out previously, theseadvantages are achieved without any significant loss of the highlydesirable structural strength properties associated with a polyimidederived from methylenebis(phenyl isocyanate) alone. This finding isparticularly surprising in view of the markedly lower structuralstrength properties possessed by polyimides derived from toluenediisocyanate alone.

The copolyimides of the invention can be employed for any of the uses towhich high temperature resistant polyimides are currently put in theart, for example, the copolyimides of the invention can be molded in theform of bushings, seal faces, electric insulators, compressor vanes andimpellers, piston rings, gears, thread guides, cams, brake lining,clutch faces, abrasive articles and the like. In solution form they canbe employed in the preparation of polyimide coating compositions and canthereby be employed in wire coating and in the casting or spraying ofpolyimide films on a variety of substrates such as metal, ceramic,fabrics, polymerics and the like.

Indeed, the copolyimides of the invention being soluble in organicsolvents represent a particularly useful advance in the art since theyprovide, for the first time, a means of fabricating high temperatureresistant polyimides without the need to carry out a final chemicalreaction to produce the polyimide in situ. Thus, in order to producepolyimide coatings having useful high temperature resistant propertieson a variety of substrates such as wire, fabrics and the like, or toapply polyimides as high temperature resistant adhesives for metals andthe like, it has hitherto been necessary to use a solvent solublepolyimide-forming precursor which is applied in organic solvent solutionas a coating or the like and is then coverted in situ to the desiredpolyimide by heat treatment, or chemical treatment and the like; see,for example, US. Pat. No. 3,179,630.

In contrast, the soluble copolyimides of the invention can be applieddirectly as a coating or adhesive and re moval of the carrier solvent isthe only operation to be accomplished after application. Further,because of the thermoplasticity of these copolyimides, shaping ormolding of the coated material can be accomplished after removal ofsolvent. This is of particular advantage in the preparation of laminatesand the like from fabric and like materials coated with the copolyimidesof the invention.

Reinforced composites made by incorporating fi brous reinforcements intothe copolyimides of the invention can be manufactured by lamination,coating, molding and other methods known to those skilled in the art.

The fibrous reinforcements can include those produced from inorganicmaterial such as quartz, metal,

glass, boron, or graphite fibers, or organic materials like aromaticpolyamides, polyimides, polyamideimides, or other high temperatureresistant fibers. The fibrous reinforcements can be in the form offilaments. yarn, roving, chopped roving, knitted or woven fabrics.

In a practical embodiment, a plurality of layers of a woven fibrousreinforcement impregnated with a solution of a copolyimide of theinvention from which a major portion of the solvent has been removed,can be brought together in a heated press and formed into a truelaminate.

In another embodiment, a plurality of layers ofa solid copolyimide ofthe invention in which is embedded roving or yarn, formed by removingthe solvent from a solution of a copolyimide mixed with the roving oryarn, can be brought together in a heated mold and formed into areinforced composite.

In yet another embodiment, roving which has been coated with a solutionof a polyimide of the invention and the solvent removed, can be choppedinto pieces, placed in a heated mold and formed into a reinforcedcomposite. A variation of this embodiment consists of molding anintimate mixture of chopped roving, yarn, or filament and a powderedcopolyimide of the invention to form a reinforced composite. Othertechniques of fabricating reinforced composites using the copolyimidesof the invention will be readily apparent to those skilled in the art.

The solvent soluble polyimides of the present invention exhibit superiorheat resistance as compared with the soluble polyimides of the aboveidentified copending application Ser. No. 124,958, as evidenced by themuch higher Tg values (e.g., 367C) of the copolyimides of this inventionwhen compared to the maximum of 304C for the copolyimides in the abovecopending application.

The organic solvent soluble copolyimides of the invention also showadvantages over high temperature resistant polyimides hitherto known inthat their properties enable them to be used to produce articles havingreinforcing or modifying tillers and the like incorporated therein.Thus, fillers such as fiberglass, carbon fibers, graphite, molybdenumdisulfide (to impart lubricity), powdered metals such as aluminum,copper and the like, and abrasive materials (for producing grindingwheels and the like) can be added to solutions of the solublecopolyimides of the invention and intimately mixed therewith prior toremoval of solvent followed by heat pressing or like techniquesnecessary to achieve production of the desired article. Other processingadvantages which accrue from the high temperature resistance, solventsolubility and thermoplasticity of these copolyimides of the inventionwill be apparent to one skilled in the art.

The following examples describe the manner and process of-making andusing the invention and set forth the best mode contemplated by theinventors of carrying out the invention but are not to be construed aslimiting.

EXAMPLE 1 A 1.5 liter resin kettle equipped with a reflux condenser,thermometer, mechanical'stirrer, constant ad dition funnel, and aside-arm distillation (no condenser) apparatus, was set up in an oilbath.

750 ml. of dry N-methylpyrrolidone (NMP) was placed in the kettle alongwith 87.25 g. (0.4 mole) of pyromelli i di nhy and heated o C- 1 wasisolated by extruding the hot NMP solution into The dark brown solutionwas then placed under vacw (72C) water, Th s lting solidified rope-likeuum g-) causing 30 OfNMP and a trace strands were ground in a WaringBlender with hot quantity of water t0 d till Of water and washed for 2hours, collected by filtration A So 0f55.7 gmole, of toluene 5 andwashed twice again. The final slurry was filtered, isocyanate (TDI: pure2,4iS0me and 20. gallowed to air-dry overnight and then vacuum dried atmole, 20%) of methylenebis(phenyl isocyanate) (MDI) 180C f 16 2() h wascharged to the addition funnel. It was added slowly TGA l i f h polymerh d a 13% i h to the PMDA S l ion Ov r a 4-5 h r Period at loss due towater but no further loss until 300C. Geh- Carbon dioxide gas bubbleswere evident on the surman curve analysis (plot of torsion moduli vs.temperaface within one hour of the addition. Stirring and heatture, ASTMDl053-65) showed a softening to a low ing were continued and 0.5 hourafter completion of value of yield point of 425C. While the polymer wasthe NCO addition an infrared spectrum showed the soluble in the reactionsolution, once isolated it could presence of small amounts of NCO andanhydride. An not be redissolved in NMP or dimethyl sulfoxide additional100 ml. of NMP was added to reduce solu- 15 (DMSO).

TGA Analysis Wt. Loss (in air) 100C 200C 300C 400C 450C 500C 550C tionviscosity with heating at 80C continued. One hour 7 EXAMPLE 3 after NCOaddition was completed, the infrared analy- The apparatus described inExample 1 was charged sis indicated only trace amounts of NCO andanhywith 65.4 (0.3 mole 75%) of pyromelhtic dianhy dride. Another 100ml. pOl'tiOl'l Of NMP was added 110 dride and 322 g (01 mole 25% f33I,4,4I reduce Viscosity and heurs after NCO addition had benzophenonetetracarboxylic acid dianhydride. The been completed the teacher wasStopped The final mixture was dissolved in 750 ml. of NMP forming apolymer solution concentration was 12.0%; m (1.0% dark brown solutionwhich was heated to by in NMP) 0.83. There was thus obtained a solutionin means of an Oil bath By reducing the pressure to N'methylpyrrolidoneof a coPolyimide characterized proximately 1-2 mm Hg, 50 ml. of solventplus moisy a recurring unit of the formula ture was taken off overhead.

. A solution of 48.8 g. (0.28 mole, 70%) ofTDl in 25.0

g. (0.10 mole, 25%) of MDI was charged to the addi- 3 tion funnel (thisconstitutes 95% of the theoretical NCO). The isocyanate was slowly addedover a 6 hour period, rinsing the addition funnel clean with 25 ml. of

E E 4 NMP which was added to the reaction mixture. The re- 0 action wasallowed to proceed overnight while maintaining the temperature at 80C.

A second addition funnel was charged with 5.6 g. (0.028 mole) ofmethylene dianiline dissolved in 35 ml. of NMP. This was equivalent tothe 5% MDI deficiency plus a 2% excess. It was added slowly so thatafter 3.5

hours, when approximately 70% was consumed, reaction viscosity was veryhigh. The reaction mixture was diluted with 400 ml. of NMP which reducedthe origiand wherein in 20 Percent of the units represents nalconcentration from 15.5% down to 11.1%. Temperature was increased to98C. Further addition of MDA was limited to only 0.5% excess due toviscosity.

Ring closure of the amic-acid fraction was accom- Q plished bydistilling 50 ml. of combined NMP and moisture from the kettle underreduced pressure. Continued stirring at 98C for 2 hours reduced theviscosity. Infrared analysis still showed some anhydride present. Morewherein R represents and in the remaining 80 percent of the unitsrepresents cg, MDA was added to a total of 3.5% excess and more NMP (130ml.) was stripped off under vacuum at 98C. The final reactionconcentration was 12.4%.

A portion of the reaction solution which was allowed to cool to ambienttemperature had an average m 1.41 after 3 days standing. A sample of thepolymer so- Laminates and films were prepared from the polymer lutionstored at 80C for 3 days had an average m solution by standardtechniques. 0.75. The room temperature solution did not precipitate orgel even after 2 months standing. EXAMPLE 2 The solid polymer wasisolated from its hot solution The copolyimide prepared as described inExample by precipitation in warm water, washed, collected, then washedtwice again and finally dried at 200C under vacuum. There was thusobtained a copolyimide characterized by a recurring unit of the formula:

wherein R in 75 percent of said units represents and, in the remainingpercent, represents and wherein R in 70 percent of said units representsand, in the remaining percent, represents Gag Once isolated fromsolution, the copolyimide would not redissolve either in NMP or DMSO. A1% solution in concentrated sulfuric acid had an m 0.12.

The polymer powder was easily molded at 395C at 25,000 psi. Theresulting test bars showed a Tg of 367C by Gehman analysis versus a Tgof 382C by Thermomechanical Analysis which is remarkably good agreementfor the determination of a second-order transition by two differentmethods. The test bars showed an average flexural strength of 18,850 psiand flexural modulus of 419,000 psi.

EXAMPLE 4 The apparatus described in Example 1 was charged with 65.4 g.(0.3 mole, 75%) of PMDA and 32.2 g. (0.1 mole, 25%) of BTDA dissolved in905 ml. of NMP (dried over 3A molecular sieves). After heating the darkcolored solution to 80C by an oil bath, 129 ml. of NMP was slowlydistilled out under reduced pressure (4-5 mm. Hg. over a 3 hour period)until the total NMP volume was 776 ml. (15% conc.).

A solution 48.8 g. of TDl (0.28 mole, 70%) and 30.0 g. (0.12 mole. 30%)of MD] was charged to the addition funnel. The anhydride solution wassaturated with carbon dioxide, a wet test meter was zeroed, and the slowaddition of the NCO solution was initiated at 80C. Over a 5.75 hourperiod the addition was complete with 10 ml. of NMP used to rinse theaddition funnel into the reaction mixture. Following this, 60 ml. of

solvent was stripped from the mixture under reduced pressure over anhour to result in a 16% reaction concentration. The stripped NMP showeda trace of moisture by observing the 2.95 1. peak absorhance differencefrom the sieve dried material. The reaction mixture was allowed to stirovernight.

An increase in viscosity was observed and infrared showed no NCOremaining but a trace of anhydride. A second NCO solution was preparedfrom 0.8 g. 1 mole of MDI in 10 ml. of NMP and a 0.1% excess was addedand allowed to react. The solution was diluted with NMP to achieve a 14%concentration.

A sample of the reaction mixture was diluted to 1.0% from 14.0% and hadan average m 1.05.

There was thus obtained a solution in N- methylpyrrolidone of acopolyimide characterized by the recurring unit of the formula:

wherein R in 75 percent of said units represents and, in the remainderof said units represents and wherein R in percent of said unitsrepresents cit and, in the remainder of said units represents Films werecast using normal casting and drying procedures.

EXAMPLE 5 The apparatus described in Example 1 was charged with 65.4 g.(0.3 mole, of PMDA and 32.2 g. (0.1 mole, 25%) of BTDA dissolved in 905ml. of NMP. The colored solution was brought to C by means of an oilbath and 130 ml. of NMP was slowly distilled out under reduced pressure(3 mm. over a 2 hour period) until the total NMP volume was 775 ml. (15%concentration).

A solution of 62.75 g. (0.36 mole, of TDI and 10.0 g. (0.04 mole, 10%)of MDl was charged to the addition funnel. At a reaction temperature of80 C, the NCO solution was added over a 6.0 hour period and 10 ml. ofNMP used to rinse the addition funnel into the resin kettle. A 60 ml.portion of solvent was removed from the reaction mixture under reducedpressure at 80C which resulted in a 16% concentration of polymer insolution.

An additional 1% excess of NCO was added as a solution of 0.63 g.(0.0036 mole) of TDI and 0.10 g. (.0004 mole) of MD] in 60 ml. of NMP asa carrier. The addition took 1 hour and the reaction mixture was heatedat 80C for an additional 2 hour period.

The solid polymer was isolated by extruding the hot NMP solution intowarm water. The solidified rope-like strands were ground in a WaringBlender with hot water and washed for 2 hours. The solid was collectedand washed twice again in water. The final slurry was filtered, allowedto air-dry and then vacuum dried at 180C for 20 hours. There was thusobtained a copolyimide characterized by the recurring unit of theformula:

wherein R in 75 percent of the recurring units represents and, in theremaining 25 percent represents and wherein R in 90 percent of therecurring units represents and, in the remaining percent, representsThis solid polymer was very easily molded due to the high TDlconcentration.

EXAMPLE 6 For purposes of comparison a polyimide of the prior artderived from PMDA and TDl alone, was prepared as follows: A l,500 ml.resin kettle equipped with a reflux condenser connected to a gas inlettube, a long stem thermometer, a l25 ml. addition funnel, a mechanicalstirrer, and a gas capillary tube was flamed out under vacuum andflushed with dry CO A combination of 695 ml. of NMP (dried over 3Amolecular sieves) and 87.25 g. (0.4 mole) of PMDA was charged to thekettle and, after stirring, the reddish-yellow solution was heated to100C while being saturated with CO A solution of 69.67 g. (0.4 mole) ofTDl (pure 2,4 isomer) dissolved in 55 ml. of NMP was charged to theaddition funnel. The CO gas flow was terminated and a wet test gas meterconnected to the condenser was zeroed. A reaction temperature of l00Cwas maintained while slow addition of the NCO solution was completedover a 5 hour period. After 92% of the NCO had been added, a precipitatebegan forming and shortly after completion of the NCO addition, theviscosity build-up prevented stirring of the mass and heating wasstopped. The reaction mixture formed a firm gel.

The semi-solid gelatinous material was ground up with 2 liters of waterin a Waring Blender for 3 hours causing the formation of a filterablesolid. It was washed a second time in the blender, collected on asuction filter, air dried, followed by thorough drying at 200C under 0.5mm. pressure for 24 hours.

The resulting copolyimide was characterized by a single recurring unitof the formula;

Viscosity measurements of the polymer were not possible due to itsinsolubility.

EXAMPLE 7 A 10 ply polyimide fiber glass laminate was prepared by firstbrushing a 30% solution of PMDA-/20 TDI/MD] polyimide dissolved in NMP(prepared as described in Example 1) on one side of a single layer of181E (A-l l00)* fiber glass cloth held in tension on a hand coatingmachine. Solvent was almost totally removed by infrared heaters and thecloth turned over and the polyimide solution applied to the oppositeside. Again the solvent was removed to yield a fiber glass prepreg witha pliable coating of polyimide on both sides. Using this prepreg cloth,any number of layers may be laminated, but in this specific example 10layers were prepared using the following procedure.

* l 8lE (A-l is a fiber glass cloth supplied by Burlington Glass FabricsCo. and E designates the type of glass and l8l designates the weave; A-lI00 is B-aminopropyltn'ethoxysilane supplied by Union Carbide and actsas a coupling agent between the glass cloth and the polymerbeingapplied.

Ten layers of prepreg were placed in a heated press and carried througha first cycle at 350-400F at a maximum pressure of 500 psi which wasallowed to reduce to 0 psi over 4% hours. The second cycle was 7 hoursat 400-620F under 200-500 psi. The resulting laminate showed excellentstrength retention under high temperature conditions as shown in thefollowing test data.

A ply polymide graphite cloth laminate was prepared using the proceduredescribed in Example 7, Hitco G((] 1550)* graphite cloth being used inplace of fiber glass cloth. Ten individual graphite cloth prepreg layerswere laminated using the same two cycle procedure outlined in Example 7.The resulting laminate showed excellent strength retention under hightemperature conditions (2500 F).

*Hitco G(G 1550), a product of Hitco Co., is a graphite cloth having 99%minimum carbon content with a fiber diameter of 0.0003 Inch.

We claim: i

1. A heat resistant reinforced composite comprising a mixture of (1) acopolyimide having the recurringunit wherein R in 75 100 percent of saidrecurring units represents and, in the remaining 0 25 percent of saidunits, represents where X is a member selected from the group consistingof CO, 0, and S0 wherein R in 10 35 percent of said recurring unitsrepresents and in the remaining 65 percent of said units represents amember selected from the group consisting of CH3 ca and mixturesthereof, and (2) a fibrous reinforcement material.

2. A composite according to claim 1 in which the fibrous reinforcementmaterial is a glass fabric.

3. A composite according to claim 1 in which the fibrous reinforcementmaterial is a graphite fabric.

4. A composite according to claim 1 in which the fibrous reinforcementmaterial is a high temperature resistant organic fabric.

1. A HEAT RESISTANT REINFORCED COMPOSITE COMPRISING A MIXTURE OF (1) ACOPOLYIMIDE HAVING THE RECURRING UNIT
 1. A heat resistant reinforcedcomposite comprising a mixture of (1) a copolyimide having the recurringunit
 2. A composite according to claim 1 in which the fibrousreinforcement material is a glass fabric.
 3. A composite according toclaim 1 in which the fibrous reinforcement material is a graphitefabric.